Rationale

The midcontinent region of the United States contains four known world-class Mississippi Valley districts and several smaller ones, and large areas of permissive host rocks—i.e., Paleozoic sedimentary carbonate rocks. General descriptions of the Mississippi Valley deposit type are in Heyl (1982); Heyl and others(1959); Pratt (1982); Sangster (1983); and Briskey (1986).

On the delineation of permissive tracts

The permissive tract is defined simply by the known or inferred presence of significant sections of sedimentary carbonate rock of Cambrian through Mississippian age occurring within 1 km of the surface. The permissive tract is bounded on the east by a common boundary with tract EC03, positioned over the western flanks of the Cincinnati Arch and Nashville dome. The southern and western boundaries are defined by burial of permissive carbonate rocks beneath 1 km of cover. The northern boundary is defined by the boundary between the Great Plains and Lake Superior Provinces and, in the region of the Michigan Basin, by burial. A small area of permissive terrane is also found surrounding the Llano uplift in central Texas. Enclaves of non-permissive terrane occur in the central Illinois Basin due to burial and in the vicinity of St. Francois Mountains and center of the Llano uplift due to absence of host carbonate rocks.

Important examples of this type of deposit

The principal Mississippi Valley districts in the Mid-continent are the Southeast Missouri lead-zinc-silver-copper district (subdivided into the Old Lead Belt and the Viburnum Trend), the Tri-State zinc-lead district (Kansas-Oklahoma-Missouri), the Upper Mississippi Valley zinc-lead district (Wisconsin-Iowa-Illinois), and the Illinois-Kentucky fluorspar-lead-zinc district (in which the fluorspar had a magmatic source and is atypical of Mississippi Valley mineralization; thus the district is a hybrid). Less important are the Northern Arkansas zinc district and the Central Missouri lead-zinc-barite district. In this assessment the potential for districts is considered rather than individual deposits, because the important Mississippi Valley deposits in the Central Region are the products of broad, pervasive hydrothermal mineralizing systems, and the mineralized areas are so interconnected that their designation as individual "deposits" may be only a matter of the location of property lines. The Mississippi Valley grade and tonnage models are themselves based on data for entire districts and not individual deposits.

Rationale for Numerical Estimates

Two basically different approaches were used in this step of the assessment. These are referred to here for convenience as the Pratt method and the Goldhaber-Leach-Viets method.

Pratt Method

This method is limited in geographic extent to only a part of the Central Region, and uses a more restricted model than the other approach—a simple Mississippi Valley occurrence model in which the critical features are: (1) proximity to the limestone-dolostone interface (and preferably on the dolostone side thereof); (2) "windows," edges, or other gaps in underlying shales; (3) presence of overlying shale caps; and (4) additional data such as favorable structure or known mineral shows. (These data on limestone-dolostone ratios, shales, etc., were obtained from maps recently compiled by Pratt for the Mid-continent Strategic and Critical Minerals (MSCM) project, specifically for the purpose of identifying prospective areas for Mississippi Valley deposits.) This regional analysis led Pratt to identify some 16 prospective areas with various combinations of the model criteria. To these 16 areas were then added 8 other areas that had been assigned at least a "high potential" for undiscovered Mississippi Valley deposits in the five recent CUSMAP studies in this region (Rolla quadrangle—Pratt and others, 1984; Springfield quadrangle—Erickson and Chazin, 1991; Joplin quadrangle—Pratt and others, 1993(a); Harrison quadrangle—Pratt and others, 1993(b); Paducah quadrangle—Goldhaber and others, 1992).

In the present assessment, Pratt assigned probabilistic estimates to the existence of a new median-size district in each of the identified prospective areas within the MSCM quadrangle, excluding those that would be considered extensions of known districts; Goldhaber did the same for the two prospective areas in the Paducah 1° x 2° quadrangle. The results of the probabilistic estimates when summed give an expected mean value of about 0.2 of a district. At the 90th, 50th, 10th, 5th, and 1st percentile, this mean is consistent with estimated values, respectively, of 0, 0, 0, 2, and 4 undiscovered Mississippi Valley districts in the MSCM quadrangle consistent with the grade and tonnage model for Southeast Missouri Pb-Zn districts as defined by Mosier and Briskey (1986) (Mark3 index 42). Areas of permissive terrane outside the MSCM quadrangle are not assessed in this method.

Goldhaber-Leach-Viets Method

In this approach the three proponents, applying their combined general knowledge of Mississippi Valley deposits and their processes of formation, both in the Mid-continent region and worldwide, examined the permissive tract, and outlined areas believed to be favorable on the basis of four factors: (1) known presence of typical sulfide and gangue minerals, (2) known extent of a typical hydrothermal system, (3) proximity to fold belts and arches, and (4) bleaching of underlying basal (originally red) sandstone. The assessment team specifically opted not to attempt the analogue type of estimate—i.e., how many districts of size x would fit into permissive tract—because of the insufficiency of data.

The group reached consensus for the 90th, 50th, 10th, 5th, and 1st percentiles, respectively, of 2, 3, 3, 6, and 8 undiscovered deposits. This estimate was anchored at the 90th percentile, on a subjective agreement that at least one undiscovered lead-dominant district and one undiscovered zinc-dominant district, consistent with the revised grade and tonnage model for Southeast Missouri Pb-Zn districts as defined by Mosier and Briskey (1986), exist somewhere in the region; and that the availability of favorable areas limits the range to a maximum of 8 districts at the 1st percentile. The mean expected district value for this distribution range is about 3 undiscovered districts consistent with the grade and tonnage model for Southeast Missouri Pb-Zn and Appalachian Zn districts as defined by Mosier and Briskey (1986) (Mark3 index 108).

Non-reconciliation of the Two Assessments

No attempt is made to reconcile the two assessments because the areas considered are not directly comparable and because different assessment methods were used, one considerably more restrictive, detailed, and site-specific (Pratt Method) than the other. For purposes of consistency with the assessment philosophy followed in other assessments the editors have adopted the results of the Goldhaber-Leach-Viets Method for calculation of commodity endowments as they reflect consideration of the entire permissive tract.